1. Field of the Invention
The present invention relates to a driving method and a driving device for driving an LED element having therein a plurality of light emitting layers different from each other in light emission wavelength peak, and also to an illumination apparatus and a display apparatus.
2. Description of the Related Art
As techniques on III-V compound semiconductors and organic compound semiconductors progress, illumination apparatus have been proposed that use light emitting diodes (LEDs) made of those materials. However, because ordinary LED elements are high in purity of color of emitted light, it is difficult to obtain a color low in chroma, suitable for an illumination apparatus, such as white, only by an LED element having therein a layer or layers for emitting light of a single color. For this reason, an illumination apparatus has been devised that uses LED lamps in each of which three kinds of LED elements for red (R), green (G), and blue (B) are provided within one package and the three colors are mixed to emit white light as in general illumination. Another illumination apparatus has been also devised that uses LED lamps in each of which there are molded an LED element for emitting a short-wavelength light such as blue or ultraviolet light, and a fluorescent substance to be excited by the short-wavelength light to emit white light.
In case of the former illumination apparatus, however, because the LED element for emitting red light is made of a GaAs-base compound material, its As ingredient causes heavy environmental load. In addition, in case of the former illumination apparatus, each LED lamp includes therein three kinds of LED elements that differ from one another in base material and thus differ from one another in the manner of change in characteristic in response to a change in the surrounding environment, such as temperature, or due to aging. As a result, the LED lamp is apt to vary in color tone. On the other hand, the latter illumination apparatus is inferior in the point of light emission efficiency because it utilizes wavelength shift by a fluorescent substance. In addition, it is apt to vary in color tone because the change in characteristic of the LED element and the change in characteristic of the fluorescent substance in response to the surrounding environment or due to aging do not match each other.
In order to eliminate those disadvantages, development of an LED element capable of emitting white light by a single chip, as disclosed in JP-A-11-121806, is being advanced. FIG. 15 shows a schematic view of the LED element disclosed in JP-A-11-121806. In the LED element, as shown in FIG. 15, three light emitting layers 103, 105, and 106, made of indium gallium nitride (InGaN), are put on each other with being separated by barrier layers 104. The light emitting layers 103, 105, and 106 differ from one another in light emission wavelength peak, and emit lights in the red, green, and blue regions, respectively. The above-described five layers are sandwiched by an n-type current injection layer 102 formed on a substrate 101, and a p-type current injection layer 107. Electrodes 108 and 109 are formed on the p-type and n-type current injection layers 107 and 102, respectively.
In the LED element, when a current is made to flow between the electrodes 108 and 109, three colors of red (R), green (G), and blue (B) are mixed to emit white light. Further, because each of the light emitting layers 103, 105, and 106 is made of InGaN, various color tones can be realized by controlling the light emission wavelength peak of each light emitting layer within the range from the ultraviolet region to the red region. If LED lamps each including the LED element disclosed in JP-A-11-121806 are used for an illumination apparatus, the above-described disadvantages will be eliminated. In addition, a merit will be obtained that each LED lamp has a simple structure including only one LED element and no fluorescent substance.
Characteristics of the LED element disclosed in JP-A-11-121806 have not yet been sufficiently studied. Thus, even if the LED element is intended to be used for an illumination apparatus or a display apparatus, no technique for effectively driving the LED element has been known.